Emerald Lake's dam consists of three main components: the embankment, the principal spillway, and the emergency spillway. According to the 1999 National Dam Inventory, the original 33-foot tall embankment was constructed in 1946. Improvements have been made to the dam's embankment and spillways since that time.
Emerald Lake’s principal spillway's intake is submerged several feet below the water’s surface, and the outfall is located at the base of the embankment on the dry side of the dam (see graphic below). The pipe is equipped with a small vertical vent at the crest and a valve at the outfall, both of which are open during normal operation. This allows passive flow of water out of the lake to maintain a safe vertical distance between the normal water level and the top of the embankment. This safety margin on the dam is called the freeboard.
The crest of the pipe, where it passes through the embankment, is the lake’s designed normal or “full pool” elevation. When the water level is below the crest of the pipe, no water is released from the lake and any further drop in the water level is a result of evaporation, absorption, and authorized irrigation activities. As the water level rises above the pipe’s crest, water is released downstream until the level drops back to the “full pool” elevation. The emergency is designed to start discharging as the water level exceeds 6 inches above the "full pool" capacity.
Maintaining the "freeboard" through proper operation of the principal spillway is essential to the long-term safety and lifespan of the dam. "Over topping” from extreme rainfall is one of the leading causes of catastrophic collapse of embankment dams, and it accounts for 34% of all dam failures in the U.S. For this reason, the valve on the principal spillway should never be closed to raise the water level to satisfy navigational access or irrigation use of the lake.
Pursuant to Article X, Section 25 of the restrictive covenants, Plainsmen Developments, Inc. reserves the right to temporarily drain the lake at any time (pending 30 days written notice to the lot owners) for no more than 90 days if avoidable, for repairs and/or modification to the dam or dredging activities. The lake level may also be lowered once per winter to facilitate dock construction and/or shoreline cleanup by waterfront property owners.
The principal spillway is designed to be used as a siphoning system to draw down the water level when desired. The siphoning action is triggered by closing the vent and evacuating all air out of the system. As long as the vent remains closed, siphoning will continue until the discharge valve is manually shut or the water level reaches the depth of the intake and suction is lost.
The maximum drawdown level is approximately 4 to 5 feet vertical below the designed "full pool" water level. Chewacla Creek will continue to flow into the lake throughout the drawdown process. The length of time it takes to drain and refill the lake depends on precipitation and therefore is unpredictable.